In the transformer coil art, a number of methods have been adopted for holding or anchoring the turns of an electrical coil, so that they can resist movement when the turns are subjected to the flow of current and consequent electromagnetic forces tending to move them out of position. A commonly practiced method is to bond the turns of the coil to the layer insulation by the use of resinous adhesive layers or resin solution impregnated layer insulation. This method has not been entirely satisfactory, since by filling the coil with such continuous adhesive layers or completely resin solution impregnated materials, it is rendered impervious to the penetration of oil which is essential in providing high impulse strength in transformers.
To provide improved porous solidification of transformer coils, which will withstand large surges of power with resulting high mechanical stresses, Ford, in U.S. Pat. Nos. 3,237,136 and 3,246,271 used discontinuously patterned resin solution impregnated Kraft paper as the restraint. This method averages about a 0.25 mil to 1.5 mil adhesive thickness build. Resinous solutions of shellac or epoxy resin, to cover from 25% to 75% of the Kraft paper sheet, were taught.
This coating with resin solution covers the paper fibers under the adhesive pattern. When the patterned paper is subjected to a high humidity atmosphere, the paper surrounding the adhesive pattern can swell such that, in some cases, the adhesive pattern forms a depression and is rendered ineffective to bond coils. Problems of resin solution running outside of the patterned area were also encountered. Additionally, these resinous solutions usually utilized all organic glycol ether solvent carriers, such as methyl Cellosolve or ethyl Cellosolve, or other organic solvents which could pose health problems, and which required costly incineration of the displaced solvents used to deposit resin solids.
Other methods, such as those of Croop et al., in U.S. Pat. No. 4,095,557, have provided thicker builds of adhesive patterns using resin powder particle application by electrostatic spraying through a patterned mask onto moving Kraft paper. In Croop et al., epoxy resin powder was generally preferred, allowing builds of up to 25 mils. While resin running, solution saturation and solvent problems were solved, fine epoxy dust posed a health threat, and the equipment used in the process was both complex and expensive. What is needed is a simplified, inexpensive method of making a highly porous, oil permeable insulation, having a discontinuous, clear and precise, discrete pattern about 1 mil thick, and not saturating the insulation sheet, applied in a manner to eliminate or alleviate possible resin dust or organic solvent health hazards.